While the improved papermaker's fabric of the present invention could be advantageously used in the forming, transfer, press or drying sections of a papermaking machine, for purposes of an exemplary showing the fabric will be described in its application as a forming fabric.
Several well known types of papermaking machines utilize a forming fabric comprising a fabric of interwoven warp and filling yarns formed into an endless belt. A layer of wet pulp is deposited on the forming side of the papermaker's fabric and water is withdrawn from the pulp through the fabric by vacuum means or the like located on the machine side of the fabric.
Such papermaker's fabrics can be manufactured in two basic ways. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form an endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. Both methods are well known in the art. In a flat woven papermaker's fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In a papermaker's fabric having been woven in an endless fashion, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross machine direction" refer respectively to a direction equivalent to the direction of travel of the papermaker's fabric on the papermaking machine and a direction transverse this direction of travel.
In the usual practice, papermaker's fabrics are manufactured utilizing weave patterns such as the 1/2 twill, 2/1 twill, 2/3 twill, 3/2 twill, 1/4 twill, 4/1 twill, 1/5 twill, 5/1 twill and variations thereof such as the sateens. These most commonly used weave patterns have one characteristic in common. They are all unbalanced weaves yielding two sided fabrics; one side being warp faced and the other side being filling faced. This characteristic leads to an imbalance of forces within the fabric structure. This imbalance of forces within the fabric structure, together with the different conditions which obtain at the edges of the papermaker's fabric when used on a papermaking machine (such difference including less lubrication by water, less vacuum being applied, etc.) result in a tendency of the papermaker's fabric to curl at its longitudinal edges. This tendency to curl results both in running problems and wear problems.
The present invention is based upon the discovery that if in a common unbalanced weave pattern (such as the 1/2 twill, 2/1 twill, 1/4 twill, 4/1 twill, 2/3 twill, 3/2 twill, 1/5 twill, 5/1 twill, 1/4 sateen and 4/1 sateen) selected parts of the weave are inverted, a more balanced weave is achieved and the forces within the resulting fabric are more nearly balanced. This, in turn, results in a flat fabric substantially free of edge curl and the problems attendant therewith.
Prior art workers have heretofore believed that a truly bicrimped fabric offered best sheet support and that the thinner the fabric the better its hydraulic action. Surprisingly, the weave patterns of the present invention eliminate pairing and yield a more bicrimped-like fabric. The forming side of the fabric displays improved sheet support and better sheet release. Sheet support is obtained from both yarn systems. The better sheet support is achieved through structural design rather than by providing a truly bicrimped fabric.
The fabric of the present invention has the same number of knuckles on both side and therefor is balanced. It is also thicker with greater internal volume, and yet it runs cleaner and displays better drainage. At the same time, the machine side of the fabric is characterized by substantially no reduction in wear quality.
The improved uniform surface support and the greater internal volume of the fabric of the present invention would also be advantageous when used in the transfer, press or dryer sections of a papermaking machine.